1. Technical Field
The present invention relates to a method and system for determining the efficiency of a catalytic converter based on signals generated by pre-and post-catalyst exhaust gas sensors.
2. Background
As is known in the art, increasingly stringent federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy. To meet these requirements, it is necessary to monitor the performance of the catalyst using an on-board catalyst monitor. These monitors are designed to meet the on-board diagnostics II regulations (OBDII) that state that an automobile manufacturer must be able to determine when the performance of the catalyst has deteriorated to the point that the vehicle is emitting 1.75 times the regulated limit of pollutants. Catalytic converters are often used to reduce emission levels of regulated exhaust gases. The conversion efficiency of a catalytic converter may be monitored using a pre-catalyst oxygen sensor (HEGO sensor) positioned upstream from the catalytic converter and a post-catalyst oxygen sensor (catalyst monitor sensor or CMS) positioned downstream from the catalytic converter.
One method known for indicating conversion efficiency of the catalyst is to calculate a ratio of the accumulated number of CMS transitions or switches to the accumulated number of HEGO transitions or switches. An increasing switch ratio is generally indicative of a degrading catalyst. When the switch ratio exceeds a threshold value, a malfunction indicator light (MIL) is illuminated to alert the vehicle operator.
Another known method for indicating conversion efficiency of the catalyst determines a ratio based on an accumulated per sample change in magnitude of CMS sensor voltage relative to a corresponding change in magnitude of the HEGO sensor voltage.
As is also known in the art, modern gasoline engines are operated by oscillating, the air fuel ratio around the stoichiometric composition for combustion at a fixed frequency. In general, a catalyst that is functioning at high efficiency will significantly damp any oscillations in the amount of oxygen in the exhaust that passes through the catalyst and hence the signal from the rear sensor will be different than the signal from the sensor on a normally functioning catalyst. Numerous mathematical comparisons of the two signals can be made. One particularly useful method is to compare the ratio of the path length of the signal from the rear sensor to that of the front sensor. Such method is described in U.S. Pat. No. 5,899,062 entitled xe2x80x9cCatalyst Monitor Using Arc Length of Pre-and Post-Catalyst Sensor Signalsxe2x80x9d Inventors Robert Joseph Jerger, Christopher Kirk Davey, Michael I. Kluzner and David R. Nader, issued May 4, 1999 assigned to the same assignee as the present invention, the entire subject matter thereof being incorporated herein by reference. As described in such U.S. Patent, and referring to FIGS. 1A-1C, representative voltage signals generated by an upstream 32 (FIG. 1A) and downstream sensor (FIGS. 1B and 1C) are shown. FIG. 1A illustrates an upstream voltage signal from an upstream sensor, here a heated oxygen sensor (HEGO), as a function of time. It is noted that the upstream sensor signal 40 oscillates through a switch point of 0.45 volts between a high voltage and a low voltage in response to the combustion mixture oscillating about the stoichiometric ratio during closed loop control.
FIG. 1B shows a representative downstream voltage signal 42 provided by a downstream sensor as a function of time for a catalyst having relatively high conversion efficiency. Although the frequency of downstream sensor signal is the same as the frequency of upstream sensor signal, the downstream sensor signal has a much different variation in amplitude than upstream sensor signal and is phase shifted due to the propagation delay of exhaust gases passing through the converter and associated exhaust piping. The amplitude variation or excursion of downstream sensor signal (FIG. 1B) is much less than that of upstream sensor signal (FIG. 1A) due to the operation of the catalytic converter in converting the exhaust gases. The arc length method described in the above-referenced U.S. Patent incorporate the time-based or horizontal component of the sensor signal into the catalyst efficiency indicator. FIG. 1C illustrates a representative downstream sensor voltage signal as a function of time. In this case, the amplitude variation of downstream sensor signal is much greater than the variation of downstream sensor signal shown in FIG. 1B. A signal similar to signal to that shown in FIG. 1C results from an aged and deteriorated catalyst having low conversion efficiency. The arc length method recognizes that as the catalyst ages and deteriorates the arc length of the voltage signal provided by downstream sensor generally increases for a given period of time compared with that of an efficient catalyst over the same period of time. The arc length or path distance traversed by the voltage signal may be determined using the line integral of the voltage signal. For example, for a given period of time dL which represents the length of a infinitesimal incremental line segment on the signal, the arc length is preferably determined using any of a number of approximations for the line integral. Since signal information is collected in a discreet fashion over time, the length of the signal can be calculated with a modification of the Pythagorean theorem:             signal      ⁢              xe2x80x83            ⁢      path      ⁢              xe2x80x83            ⁢      length        =                            Δ          ⁢                      xe2x80x83                    ⁢                      t            2                          +                              A            ·            Δ                    ⁢                      xe2x80x83                    ⁢                      V            2                                          Index      ⁢              xe2x80x83            ⁢      Ratio        =                  ∑                  xe2x80x83                ⁢                              "AutoLeftMatch"                                                            Δ                  ⁢                                      xe2x80x83                                    ⁢                                      t                    2                                                  +                                                      A                    ·                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                      V                    2                                                                        "AutoRightMatch"                                downstream            ⁢                          xe2x80x83                        ⁢            senosr                                      ∑                  xe2x80x83                ⁢                              "AutoLeftMatch"                                                            Δ                  ⁢                                      xe2x80x83                                    ⁢                                      t                    2                                                  +                                                      A                    ·                    Δ                                    ⁢                                      xe2x80x83                                    ⁢                                      V                    2                                                                        "AutoRightMatch"                    upstream_sensor                    
Where xcex94t is the time between successive data samples, xcex94V is the change in sensor voltage from one data point to the next and A is a constant used to scale the relative voltage response to the time response. The ratio of the sum of the rear signal to the sum of the front signal over time is then used to calculate an index between 0 and 1 to quantify catalyst performance. A highly efficient catalyst will have an Index Ratio approaching 0 and a catalyst having a relatively low efficiency will have an Index Ratio approaching 1.
While such arc length method provides a performance measure of the catalytic converter, the inventors herein have found a method and system for further improving such arc length technique.
In accordance with the present invention, a method and system are provided for monitoring exhaust gas conversion efficiency of a catalytic converter during operation of an internal combustion engine coupled to the catalytic converter. The system includes an upstream exhaust gas sensor interposed the engine and the catalytic converter for generating a first signal based on the exhaust gas upstream of the converter. A downstream exhaust gas sensor is interposed the catalytic converter and atmosphere for generating a second signal based on the exhaust gas downstream of the converter. A phase shift detector is provided for estimating a phase shift between the first and second signals. A processor is provided for aligning the first and second signals such that the resulting phase shift between the first and second signals is substantially an integer multiple of xcfx80 radians and for determining conversion efficiency of the catalytic converter based such phase aligned first and second signals.
With such method and system, the inventors have discovered that the average error of the calculated index ratio using such system and method and the theoretical average error in index ration (IR) is minimized when the phase shift between the upstream and downstream sensor signals is an integer multiple of xcfx80 radians.
In one embodiment, the processor determined arc length of each of the phase aligned first and second signals.
In one embodiment, the processor determines an index ratio from the ratio of one of the determined arc lengths to the other one of the determined arc lengths.
In one embodiment, the processor, processes a portion of samples of the first and second signals, such portion being a function of the determined phase shift to aligning the first and second signals such that the phase shift between the first and second signals is an integer multiple of xcfx80 radians and for determining conversion efficiency of the catalytic converter based such phase aligned first and second signals.
In one embodiment the downstream sensor is a UEGO sensor.